1. Field of the Invention
This invention relates to an acoustic coating for an aircraft incorporating a frost treatment system by Joule effect, whereby said coating is more particularly designed to cover the leading edges of an aircraft and more particularly an air intake of a nacelle of an aircraft.
2. Description of the Related Art
An aircraft propulsion unit comprises a nacelle in which a power plant that drives a fan mounted on its shaft is arranged essentially concentrically.
The nacelle comprises an inside wall that delimits a pipe with an air intake toward the front, a first portion of the entering air flow, called primary flow, passing through the engine to participate in the combustion, the second portion of the air flow, called secondary flow, being driven by the fan and flowing into an annular pipe that is delimited by the inside wall of the nacelle and the outside wall of the power plant.
The noise emitted by the propulsion unit consists, on the one hand, of jet noise, produced on the outside of the pipes as a result of the mixing of various air flows and exhaust gases, and, on the other hand, of noise generated by the inside parts, so-called internal noise, produced by the fan, the compressors, the turbines and the combustion that propagates inside the pipes.
To limit the impact of noise pollution close to the airports, the international standards are increasingly restrictive as far as sound emissions are concerned.
Techniques have been developed to reduce the internal noise, in particular by using, at the walls of the pipes, coatings whose purpose is to absorb a portion of the sound energy, in particular by using the principle of Helmholtz resonators. In a known way, this acoustic coating, also called an acoustic panel, comprises—from the inside to the outside—a reflective layer, at least one alveolar structure, and an acoustically resistive structure.
Layer is defined as one or more layers that may or may not be of the same type.
The acoustically resistive structure is a porous structure that has a dissipative role, partially transforming the acoustic energy of the sound wave that passes through it into heat. It comprises so-called open zones that are able to allow acoustic waves to pass and other so-called closed or filled zones that do not allow sound waves to pass but are designed to ensure the mechanical strength of said layer. This acoustically resistive layer is characterized in particular by an open surface ratio that varies essentially based on the engine and the components that constitute said layer.
For the moment, because of various constraints, for example shaping or compatibility with other equipment, coatings are provided in particular at the inside wall of the nacelle over a limited zone that is distant from the air intake and the air discharge.
To increase the effectiveness of the acoustic treatment, one approach consists in increasing the surface areas that are covered by the acoustic coating. However, at the air intake or on the lip of the nacelle, the installation of an acoustic coating is not possible for the moment in particular because said coating is not compatible with the systems that make it possible to prevent the formation and/or the accumulation of ice and/or frost that are necessary in these zones.
These systems are divided into two families, the first called defrosting systems that make it possible to limit the formation of ice and/or frost, the second called de-icing systems that limit the accumulation of ice and/or frost and act on both the ice and/or frost formed. Hereinafter, a frost treatment system is defined as a defrosting system or a de-icing system, whereby the term frost encompasses frost or ice.
For the defrosting treatment, one approach consists in treating the aircraft on the ground by using a gas or a liquid that is deposited on the surfaces to be treated. Even if these treatments are effective, in particular at the time of take-off, they have a limited duration. It is necessary, however, that frost treatment systems be put on board the aircraft because frost can form at the aerodynamic surface of the aircraft, and, more particularly, at the leading edges of the wing, the nacelle, the stabilizer, etc., when the aircraft passes through certain meteorological conditions.